Potterton Profile Problem

[Roger]

My elderly boiler is having ignition problems.

The symptoms are that it fires up normally but over the next 10 or so seconds the gas flame starts to flicker and then goes out. Depending on whether or not the pilot light goes out as well (it varies) there is a delay before the sequence is repeated. Is the most likely problem:

a) A failing thermocouple, (which I can presumably source locally)

b) a failing gas valve (replacement where from) or

c) a failing control board (which needs replacing with a reconditioned one from Geoff)

The boiler is still producing some heat but it is struggling to get the house up to temperature while it is relatively warm outside. Something could of course fail permanently at any time and no doubt there is another cold spell round the corner when the boiler just won't be able to cope in its present state.

[adder1969]

Usually it's something like this.. controls call for heating or whatever, pcb says ok open gas valve and light the pilot. The thermocouple then tells it yep the pilot is lit so it starts the fan. The fan sensor then says yep the fan is working go ahead and open the main gas valve. If the temp gets too hot then the boiler thermostat tells it to switch off. if it gets really really hot then the overheat stat cuts in. So you need to check all the sensors valves etc in order and work out which is flaky :-)

[Roger]

The message

from adder1969 contains these words:

Thanks for that

The way I see it none of the sensors other than the thermocouple are seeing heat at that stage as the problem is there at start up. So how do I check for a flaky themocouple short of just fitting a new one?

[Ed Sirett]

Roger, seriously, let's start over on this one. There is no thermocouple on this boiler.

The black control box (available from cetltd) runs everything.

IME the most common fault on the box is that the internal relay does not acknowledge the APS. This causes the fan to stop and start repeatedly. This is not the case for you as if you have ANY flames the fan/APS and controls are good.

The box open the pilot gas valve and begins sparking. If you have ANY flames then you must have got this far. The same HT lead that lit the flames now sense that the pilot flame is good, and the box opens the main gas valve.

The problem looks like its in flame detection and/or reliable flame detection.

1) Service or get the boiler serviced. Make sure that all the part are free of crap and the HT lead is in good order and making good contact at both ends. The mains electric supply is round the right way and the earth is good. 2) If that fails to harden or remove the fault then replace the HT lead. 3) If you see the main flames even if only for a second then we know the gas valve is OK. The gas valve is easy to condemn/clear based on it's supply voltage. 4) If everything appears to be OK then replace the black box.

[geoff]

Sorry, but that's total bollocks

A call for heat powers up the pcb which, if the stat (white wires) presents a short circuit, brings on the fan, which transfers the air pressure switch from normally closed to normally open. This feeds back to the pcb which a) opens the pilot valve and b) powers the spark circuit. When the pilot flame is established, the pcb senses the flame by electronic means (there is no thermocouple), and opens the main gas valve.

[geoff]

In message , Roger writes

there is no thermocouple

could be - or faulty overheat stat, or reduced circulation for some reason

No, I don't think it's likely to be the pcb, if the boiler dies with a reasonably reliable time period

John or Ed are the people to ask here

[cynic]

Just a thought. I once (and only once) came across a gas valve with a gauze at the inlet. Said gauze was semi-clogged with dust from inside the gas pipe and when the main falme lit up it starved the pilot which became unstable and sporadically the flame sense failed. Does the flame "look" ok during this problem?

[Roger]

The message from geoff contains these words:

Thanks.

[Roger]

The message from Ed Sirett contains these words:

Thanks for the advice Ed.

Whoops! I suppose I must have known that once since I installed the boiler in the first place and have since repaired it.

I did think the spark was looking a bit feeble. I have in the past had to close the gap up (by brute force there being no means of adjustment). Could a borderline spark gap be responsible?

The fan and the HT lead have both been replaced in recent years.

I will try that if reducing the spark gap doesn't make a difference.

I hope it doesn't come to that although Geoffs prices won't break the bank.

[John Stumbles]

I had one recently where it sometimes would stick at pilot light on, no sparking (so flame sense OK) but would not pass Go or collect £200 ... or fire up the main burner. Turned out to be a dodgy solder joint between the Molex and the PCB. (Thanks for the telephone consultation Geoff and John at CET!)

[Roger]

The message from cynic contains these words:

It looks ok colourwise (well perhaps a tinge of yellow) but something is turning the gas supply off in relatively short order.

[Roger]

The message from Roger contains these words:

Well I have closed the spark gap down to the minimum (2.5 mm) and the spark looks ok but it hasn't made a blind bit of difference to my problem.

I will have to leave the detective work to another day which may not be as soon as tomorrow unless the problem gets worse as I have an urgent appointment with my boundary wall. I discovered today that a 15 foot section of the wall had collapsed into a narrow public path completely blocking it and as yet I haven't finished removing the rubble. It is a dry stone wall so will go up again once I have dug down and relaid the foundation stones but at 6 feet high there is a lot of stone to play with and only a couple of days of fine (ish) weather in prospect.

[geoff]

In message , Roger writes

Have you put a meter on the solenoid contacts to see if it is a gas valve problem or it's losing it's power ?

[Roger]

The message from geoff contains these words:

Not yet, nor even established where I can get probes in. ISTR that last time I ferreted about in the boiler electrics I had trouble with shrouded connectors.

The house was up to temperature when I got up so I am not sure whether the boiler was misbehaving this morning.

They do say that troubles come in threes and to add to the boiler and boundary wall problems I now have a domestic hot water leak. There is a puddle partly under the washing machine machine but luckily the source was a drip from above so I just have to tear up the bathroom floor rather than tear the washing machine apart.

[Andrew Gabriel]

If the flame dies down gradually, I doubt that's the boiler as it has no modulation control of the gas valve; sounds like a lack of gas. Have you got a gas hob, and if so does a lit gas burner do the same thing at the same time? If so, check the main gas c*ck is fully open, and if it is, sounds like the main regulator has died.

OTOH, if you can hear the gas valve operating simultaneously as the flames die, then it's not this.

[Roger]

The message from geoff contains these words:

I have now checked the gas valve.

Apart from the earth there are 3 wires, black, blue and white. I assume that black/blue is the 240V supply and white is the control wire.

When calling for heat there is 240V across black/blue whether the boiler is firing or not.

While the boiler is firing there are no volts on white. When it is not firing there are 240V on white.

[geoff]

In message , Roger writes

You should be able to get them into the molex connector on the pcb (pins

1 & 3 from the top I think)

[geoff]

In message , Roger writes

There is a basic diagram on the pcb cover which should give you a clue

[Roger]

The message from geoff contains these words:

T4/T5 Air pressure switch T7/T8 Fan

Sorry but none the wiser.

So is the gas valve working properly or not?

[geoff]

In message , Roger writes

I think it's T1 - T3 = main valve

T2 - T3 = pilot valve